PHYSICS OF THE AIR

1 gravity. On very warm days water vapor may amount to 5 per cent r more of the total gas molecules present, and the air, therefore, be, roughly, '2 per cent lighter than it would be if perfectly dry. Of course,' changes from saturation to utter dryness, or the reverse, do not occur in lature, but a variation of as much as 50 per cent in the absolute humidity it a given place does occur through evaporation, condensation, and air movement. Hence on very hot days a change of 1 per cent in the weight per unit volume of the lower air as a result of altered composition alone is quite possible, and indeed often occurs. This produces a difference in buoyancy of the same order as that caused by a 3° C. change in temperature and therefore may be decidedly important.

FIG. 28. — Decrease of weight due to rotation of the e;irth.

The third factor that affects weight and convection, namely, horizontal velocity, while comparatively small, occasionally may be of some importance. Its numerical value can easily bo computed. Lot NS (Fig. 28) be the axis of the earth's rotation, and lot r at latitude $ be the point under consideration. The centrifugal force / exerted by the mass m at the point P is given by the equation,

/ = wrco2,

in which to is the angular velocity of the earth's rotation, and r the distance of P from the axis. Numerically,

27T

W 86,164'

Since the mean radius of the earth is about 6368 kilometers, and since weight equals mass times gravitational acceleration, or 98 Iw, approximately, in the c.g.s. system, it follows that at latitude 40°

w

ui roim-hlv.